1. Field of the Invention
The present invention relates to a method of forming a coating film on the surface of a glass substrate, semiconductor wafer, and the like having irregularities.
2. Description of the Related Art
There are known methods of forming coating films on substrates. FIG. 8 is a graph showing the relationship between rotational speed and elapsed time in a conventional method of forming an SOG (spin-on-glass which is a solution obtained by dissolving a silicon compound in an organic solvent or a silicon oxide film formed by applying and baking this solution) film on an object to be processed in the form of a plate (hereinafter referred to as a substrate) such as a semiconductor wafer as a flattening film using a spinner. On this graph, the rotational speed (rpm) and the elapsed time (second) are shown along the vertical and horizonal axes, respectively. In the conventional method, the rotational speed of the spinner is immediately increased to approximately 5000 rpm and is maintained at that value for about three seconds in which period coating is completed.
FIG. 9 is a sectional view of major parts of a substrate showing the thickness of an SOG film formed using the above-described conventional coating film forming method. Twin patterns 101 and 102 and a global pattern 103 of aluminum or the like are formed on the surface of the substrate W shown. It is difficult for the applied solution 104 to enter the gap between the twin patterns 101 and 102, which makes the thickness T.sub.1 of the SOG film smaller than the desired thickness. Conversely, the thickness T.sub.2 of the film on the global pattern 103 becomes greater than the desired thickness. The unevenness of the film having undesirable film thicknesses necessitates an additional etch-back process to correct the film.
On the other hand, methods for applying resist are disclosed in Japanese unexamined patent publications S62-190838, S63-313160, and H2-156626 wherein a resist film having a uniform thickness is formed on a flat substrate surface.
According to the methods disclosed in those patent publications, the rotation of a substrate is separated into first and second rotational actions having different rotational speeds. These methods are not successful in forming a uniform film using an SOG solution.
A direct application of the methods wherein the rotation of a substrate is separated into first and second rotational actions to a substrate having irregularities formed on the surface thereof will not provide the desired film thickness and will therefore necessitate an additional etch-back process. This also results in the need for separately preparing a device for performing such an etch-back process. In addition, there are problems including the waste of materials because of the need for removing the SOG film which has been once formed.